One nice thing about low, constant, levels of ionizing radiation is that they actually slightly REDUCE the incidence of cancer and the like. (This is part of why Denver residents don't have horrible cancer rates compared to those living nearer sea level.) Apparently the ionizing radiation provokes the production of inducible enzymes that repair DNA and scavenge free radicals - preventing more damage from both radiation and free radicals from the cell's own energy production than the radiation causes. Up to the saturation of the induciblity it's a slight net gain. Unfortunately, the neutrino flux from fusion reactors would be too low to confer this benefit.
How's that kool-aid you're drinking? I think there isn't a strong conclusion on really low doses but that doesn't mean that they are safe.
There is evidence that pre-exposure can help with an exposure, but the pre-exposure still causes health effects. There is also in interesting NBER paper showing health effects that are higher than an LNT model would predict.